1. Field of the Invention
The present invention relates to a fluid flow rate measurement apparatus which measures the flow velocity of fluid to measure the flow rate of the fluid.
2. Description of the Related Art
Various methods and apparatuses for measuring the flow rate of air and other gases have been developed and put to practical use. With some fluid flow rate measurement apparatuses, a flow velocity or flow rate detecting element is directly disposed in a fluid flow; with other fluid flow rate measurement apparatuses, a sub-passage in which fluid flows in a branching manner is provided and the fluid flowing in the sub-passage is measured, whereby requirements for flowmeters are improved.
The flowmeter having the sub-passage has been used in diverse fields because of its superiority in measurement accuracy, measurement range, stability to pulsating flow, adjustment of backward flow sensitivity, stability of flow rate output signal, and robustness and easy handling of a detecting element.
However, if a pollutional substance exists in fluid, the pollutional substance may accumulate on a flow rate detecting element of the flowmeter. If the pollutional substance accumulates on the flow rate detecting element, characteristics of heat exchange and heat transfer between the detecting element and the fluid may be changed, and thereby characteristic change of the flowmeter will be liable to occur.
In view of the above-mentioned circumstances, JP-A-10-281838 and JP-A-10-281839 disclose a method for removing pollutional substances by subjecting a detecting element to heating at a high temperature using dedicated heating means. Further, JP-U-3-063823 discloses a method for removing pollutional substances that adheres to a detecting element by reflecting radiant heat of the detecting element using a reflector disposed in the vicinity of the detecting element.